The invention relates to arrangements responsive to polarised radio signals such as radio transmissions from satellites such as television transmissions.
In order to maximise bandwidth utilisation, signals transmitted via a satellite may be transmitted with more than one sense of polarisation; for example, signals representing one television channel may be transmitted (at a particular frequency) with one mode of linear polarisation, say, horizontal polarisation, whereas signals representing another television channel may be transmitted at the same or a near by frequency but with (in this example) vertical polarisation. Similarly, the two senses of circular polarisation can be used, respectively, instead of the two modes of linear polarisation. In order for an antenna to be set to receive signals representing a particular television channel, it is therefore necessary for the antenna to be switchable into a state in which it can receive signals having the corresponding polarisation state.